During operation of an automobile it may be desirable to bring the vehicle to a stop for a period of time, such as stopping at a traffic light, (hereafter “temporary stops”). Some vehicles may operate during temporary stops by idling an engine in an engine system. Other vehicles may stop the engine during temporary stops, even without a driver request to turn-off the engine, to reduce fuel consumption. For example, the vehicle may shutdown the engine by discontinuing fuel supply and/or ignition so that the engine spins down to rest. Then, once conditions indicate a driver's intent to launch the vehicle, the engine is automatically restarted.
During the shutdown of an engine in a vehicle, there may be air in one or more cylinders, in the engine. Air in one or more cylinders, known as “air charge”, may compress and expand in the cylinder during shutdown, acting as a spring or an air cushion. In this way, enough air charge may cause a torque on the engine and produce undesired noise and vibration. Undesired noise and vibration may lead to an uncomfortable operating experience for a vehicle operator and may increase wear and tear on the engine. Methods and systems for closing a throttle in an intake coupled upstream of the engine to lower air charge in the engine are well known in the art, for example those disclosed in international patent WO 2006/134470 A1.
U.S. Patent Application 2007/0261668 describes a further method to control noise and vibration in an engine during shutdown and to restart the engine while the engine is still in the process of spinning down. Operating to restart the engine in spin down may be in response to a change of mind (COM) (e.g., due to indications of, or changes in, a driver's intent to launch the vehicle). An intake throttle may be controlled based on increasing air charge at a point of time when a restart request is made. Air may enter the engine, fuel may be injected, and ignition may resume following the injection of air and fuel. In this way an engine may be restarted at the end of a temporary stop, upon request of the vehicle operator to launch.
However, the inventors herein have recognized various issues with such an approach. In particular, closed throttle operation during shutdown may degrade attempts to restart the engine in mid-shutdown operation. For example, there may be insufficient air in the cylinder to generate sufficient torque via combustion to return the engine to idle speed and counteract the inertia of the engine decelerating to rest, especially at low engine speeds. Additionally, by opening the throttle at the time of a restart request, there may be a lapse in time between when the restart is requested and when air charge returns to a threshold level that may be necessary for restart. In this way, there may be a substantial delay between a restart request and enabling vehicle launch.
Similarly, the engine may have already spun down to a speed range in which the engine starter, if present, is unable to properly engage the spinning engine and increase engine speed sufficiently for starting. Rather, the engine may then fully spin down and utilize a re-start from rest, which may also generate a substantial delay in providing vehicle launch relative to the driver's request.